Step 5: Change The Blend Mode Of The Rainbow Layer To "Screen"

Go up to the blend mode options in the top left of the Layers panel (it's the drop-down box set to "Normal" by default) and change the blend mode of the Rainbow layer to Screen:

The colors of the rainbow will now blend in better with the photo behind them:

Step 6: Apply The Gaussian Blur Filter

Let's blur the colors of the rainbow together so it looks more realistic. Go up to the Filter menu in the Menu Bar at the top of the screen, choose Blur, then choose Gaussian Blur:

This opens Photoshop's Gaussian Blur dialog box. Drag the Radius slider at the bottom of the dialog box towards the right to increase the amount of blurring being applied to the rainbow, keeping an eye on the image as you drag the slider so you can see a preview of what's happening. Continue dragging the slider to the right until the transitions between the colors look more soft and natural. The actual Radius value you end up choosing will depend a lot on the size and resolution of your image. For me, a value of around 50 pixels works well. Click OK when you're done to close out of the dialog box:

Here's my image after applying the Gaussian Blur filter:

Step 7: Add A Layer Mask

Click on the Layer Mask icon at the bottom of the Layers panel:

Nothing will happen yet in the document window, but Photoshop adds a layer mask thumbnail to the right of the main preview thumbnail on the Rainbow layer. If you look closely, you'll see a white highlight box appearing around the layer mask's thumbnail. This tells us that the mask, not the layer itself, is selected:

Step 8: Choose The Black To White Gradient

With the Gradient Tool still selected, right-click (Win) / Control-click (Mac) anywhere inside the document window to quickly access the gradient presets thumbnails, the same ones we saw a moment ago in the full Gradient Editor dialog box. Select the Black to White gradient by clicking on its thumbnail (third from the left, top row). Press Enter (Win) / Return (Mac) when you're done to close out of the gradient list:

Step 9: Choose "Linear Gradient" From The Options Bar

Go back up to the Options Bar and this time, click on the Linear Gradient option:

Step 10: Drag A Black To White Gradient On The Layer Mask

A real rainbow would usually appear to get brighter as it reaches higher into the sky, so we're going to give our Photoshopped rainbow that same effect by dragging a black to white gradient from the base of the rainbow (the point where the rainbow and the ground should meet) to the top of the image. To do that, with my Gradient Tool selected, I'll click my mouse somewhere along the bottom of the trees on the left. Then, with my mouse button still held down, I'll hold down my Shift key as well and drag straight up to the top of the image. Holding down Shift makes it easy to drag in a vertical direction:

When I release my mouse button, Photoshop draws the black-to-white gradient. Since the gradient was drawn on the layer mask, not on the layer itself, we don't see the actual gradient in the image. Instead, the rainbow now appears to start near the bottom of the trees on the left and gets brighter as it reaches higher into the sky: